A guide on progressing through the game

The Stages of Progression

Progression in Terraria is based on the bosses that have been defeated in the world. It is separated into two main phases, pre-hardmode and hardmode. Hardmode can further be separated into different sections based on which hardmode bosses have been defeated. This list summarizes progression:

Pre-hardmode

Pre-hardmode starts once a new Terraria world is created. A new, non-modded, Terraria world will have one world evil. This world evil comes in the form of the Corruption biome, which is a purple color, or the Crimson biome, which is a red color. These biomes can merge with other biomes, such as the desert biome, to form a mixed biome, which has its own unique enemies and loot. The world evils will naturally grow in size as time goes on, so it is important to limit their growth. During pre-hardmode, the spread of the world evil is very slow; therefore, this is a good time to take measures to limit its growth.

Corruption biome. Crimson Biome.
A sample of the Corruption surface biome is shown to the left, and a sample of the Crimson surface biome is shown to the right.

A player will typically have to fight the Eye of Cthulhu and Skeletron to make progress during pre-hardmode. Pre-hardmode ends once the Wall of Flesh has been defeated, and the message saying, The ancient spirits of light and dark have been released, appears.

Early-hardmode

Early-hardmode introduces many new enemies, armor sets, weapons, etc. One major change in early-hardmode is the addition of the Hallow biome spawning in a random location in the world. The Hallow biome can merge with other biomes, like a snow biome, similar to the way evil biomes can merge with other biomes. Also, the Hallow naturally spreads like the evil biomes, but it spreads slower than the world evils.

Hallow Biome.
A sample of the Hallow surface biome.

Once a player defeats at least one of the mechanical bosses, the Twins, the Destroyer of Worlds, or Skeletron Prime, various aspects are added to the game. There are additional enemies that spawn; the Old One's Army event upgrades to tier 2, which changes and strengthens which enemies the player faces; solar eclipses can begin to occur; and additional loot can be dropped.

Early-hardmode ends once all three mechanical bosses have been defeated, and the player receives the message The jungle grows restless...

Post-Plantera and Post-Golem

After defeating the mechanical bosses, Plantera bulbs will randomly start spawning in the underground jungle biome, which will spawn Plantera in if they are broken. Upon defeating Plantera the message, Screams are echoing from the dungeon... appears, and the player is given access to the jungle temple by a key that Plantera drops.

Once a player enters the jungle temple, there will be unique enemies, traps, furniture, and a Lihzahrd Altar. A player can use a Lihzahrd Power Cell on the Lihzahrd Altar to spawn Golem.

After defeating Golem, cultists will begin to spawn at the entrance of the Dungeon, martian probes will begin spawning, and the Old One's Army will upgrade to tier 3.

If a player kills the cultists, this will summon the Lunatic Cultist boss. Upon defeating it, the Lunar event will trigger, which will summon the Moon Lord once all Celestial Pillars have been destroyed. The Moon Lord is considered the final boss of the game, and the game can be thought of as complete after the Moon Lord has been defeated.